1. Field of the Invention
This invention relates to a system for estimating thermal stress, wherein the thermal stress generated in pressure parts of thermal equipment such as a boiler is monitored, while a load is controlled to the optimal value.
2. Detailed Description of the Prior Art
At the time of the start, stop or change in the load of a boiler, fluid temperature greatly changes, whereby a temperature difference between the metal part and the internal fluid becomes larger.
Because of this, thermal stress is generated in the pressure part of the boiler, particularly, in a nozzle corner portion of a thick-wall pressure part such as an outlet header of a secondary superheater, high thermal stress is generated, whereby the life of the thick wall pressure parts is shortened by low cycle fatigue.
On the other hand, even during steady operation, the internal pressure stress due to the internal fluid, causes creep damage of thick-wall pressure parts that accumulates.
The thermal stress generated in a thick-wall tube of the pressure part of the boiler has heretofore been estimated from the distribution of temperature in the direction of the wall thickness, and the following are known as the methods of estimating the distribution of temperature.
(1) The first one of the methods is based on an actual measured value of internal fluid temperature for estimating an inner surface metal temperature, and on a perfect thermal insulation condition for estimating an outer surface metal temperature. Using these conditions, non-stationary heat transfer equations are solved to obtain the temperature distribution of the metal part in the direction of thickness, and the thermal stress is calculated. However, in this method, it is necessary to estimate the heat transfer coefficient between the internal fluid and the inner metal surface exactly. Usually, a constant value is used as the heat transfer coefficient because of the difficulty of the exact estimation. Then the estimation accuracy of the temperature distribution in the metal is not satisfactory.
(2) Temperatures are measured on two points close to the outer surface and the inner surface of the thick-wall metal part, and the distribution of temperature in a direction of the wall thickness is approximated by a straight line between these two points to thereby measure the generated stress.
The following disadvantages are presented by the method wherein the distribution of temperature in the direction of the wall thickness is measured, which has heretofore been practised, and the generated stress is estimated on the basis of the distribution of temperature thus measured.
(1) The boundary condition of the outer surface of the metal is assumed to be for a perfect thermal insulation condition, however, in practice, there is radiation, though it is small.
Further, it is unreasonable to contemplate improving the accuracy of measuring the generated stress, because the coefficient of heat-transfer for the boundary condition of the inner surface of the metal greatly changes due to the flow rate of the internal fluid and/or evaporation conditions.
(2) When the distribution of temperature in the direction of the wall thickness is approximated by the straight line, the generated stress is underestimated to be lower than the actually generated stress.
A temperature distribution calculation cycle and a stress calculation cycle have heretofore been conducted at predetermined intervals of time irrespective of changes in the internal fluid temperature, when the stress generated in the thick-wall tube of the pressure part of the boiler is estimated from the distribution of temperature in the direction of the wall thickness.
The following disadvantages are presented by the method of estimating the generated stress, wherein the calculation cycles have been conducted at the predetermined intervals of time as described above.
(1) All of the operating conditions of a plant are assumed and the calculation cycles are determined so that the generated stress can be accurately estimated under the state where the change is largest.
In other words, even at the time of steady operation where fluctuations in the fluid temperature are small, the generated stress is estimated in the same calculation cycles as those at the time of the start, stop or the load change, whereby the accuracy is in error at the time of the steady operation.
(2) In general, at the time of the start, stop or the load change in a power plant, a computer load is severe with a control computer. Calculation of the thermal stress at predetermined time intervals of time irrespective of the state of the plant is regarded as one of causes of increasing the load imposed on the computer.